Replace dns_tls with private_dns_mode setting

Also includes:
    - SettingsLib strings used in PrivateDnsModeDialogPreference
      interaction in the Settings app
    - rename ContentResolver "resolver" in methods working with
      DNS resolvers (too confusing)

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net
    - no new failures in SettingsBackupTest nor in SettingsProviderTest
    - manual interaction with developer option works
Bug: 34953048
Bug: 64133961
Change-Id: Ia7502916db9ffa0792e1e500a35e34d06a88e79d